Olefin polymerization catalysts are of great use in industry. Hence there is interest in finding new catalyst systems, including catalyst activators that increase the polymerization activity of the catalyst and allow the production of polymers having specific properties, such as high melting point and high molecular weight.
Catalysts for olefin polymerization are often based on metallocenes as catalyst precursors, which are activated either with the help of an alumoxane, or with an activator containing a non-coordinating anion.
EP-A-0 277 004 is one of the first documents disclosing polymerization catalysts comprising a bis-Cp metallocene compound that is activated by reaction with a secondary, ionic component comprising a non-coordinating anion (NCA) and a counter-cation. The NCA is, on the one hand, capable of stabilizing the active cationic catalyst species, but on the other hand only weakly coordinates to the metal center and thus can be displaced by a monomer to be incorporated into the polymer chain. The activators disclosed are ion pairs of an ammonium or phosphonium cation and a borate anion as the NCA. The cations disclosed are alkyl and/or aryl ammonium or phosphonium species. EP-A-0 277 004 states that the conjugate base of the activator cation should be a neutral component which stays in solution and which preferably does not coordinate to the metal cation. Therefore, it is suggested to use relatively bulky conjugate Lewis bases to avoid coordination to and thus interfering with the active catalyst.
WO 91/02012 discloses the polymerization of high Mw, narrow molecular weight distribution (MWD) polyethylene using a metallocene/activator system, wherein the activator comprises an NCA and an ammonium cation. Cyclic, non-aromatic ammonium cations are not disclosed.
M. A. Giardello, M. S. Eisen, Ch. L. Stern and T. J. Marks in J. Am. Chem. Soc. 1995, 117, 12114-12129, report on the preparation of cationic polymerization catalysts from metallocene compounds, using various types of activators, including methylalumoxane (MAO) and NCA/cation pairs. It is suggested that, although the main focus in activator choice is on the NCA, the choice of the amine in the activator cation may also be important, as certain amines could coordinate to the metallocene cation and thus diminish its catalytic activity. In accordance with this suggestion is another finding also reported in this article, which is that the presence of an exogenous amine base significantly depresses the Mw of the polymer product as well as the polymerization activity.
EP-A-0 630 910 teaches the use of free Lewis bases to control the activity of olefin polymerization catalysts. Lewis bases can, if necessary, terminate the catalytic reaction completely. However, the termination is reversible upon addition of alumoxane. The system disclosed thus comprises a metallocene compound, a Lewis base and an alumoxane. EP-A-0 630 910 indicates that the metallocene cation—Lewis base complex is inactive for olefin polymerization because the olefin is unable to compete with the Lewis base for the coordination site at the metal. The Lewis bases used are, among others, amines, ethers or phosphines.
Despite the earlier teachings in the art as reported above, several authors and patent applications teach the separate addition of an amine base to the system comprising the metallocene compound, the cation/NCA activator and, optionally, an organoaluminum compound. See for example U.S. Pat. No. 5,817,590, and E. A. Sanginov, A. N. Panin, S. L. Saratovskikh, N. M. Bravaya, in Polymer Science, Series A (2006), 48(2), 99-106.
U.S. Pat. No. 5,416,177 discloses the use of metallocene compounds in combination with activators comprising tris(pentafluorophenyl)borane and at least one complexing compound, such as water, alcohols, mercaptans, silanols, and oximes, for the polymerization of olefins, in particular 1-hexene. Similar catalyst systems are disclosed in WO 2005/016980.
WO 01/62764, WO 01/68718 and WO 2004/005360 disclose co-catalysts for use in olefin polymerization, wherein the co-catalysts comprise a cation derived from an aromatic, nitrogen-containing Lewis base, such as pyrrole, imidazole or indole, and NCAs, particularly borates.
WO 01/48035 relates to catalyst systems comprising an organometallic compound (preferably a metallocene), a Lewis base, a support, and an activator. The activator is an ion pair, the anion of which is a borate, such as tetrakis(pentafluorophenyl)borate. The cation is an ammonium cation, such as a trialkylammonium. No cations are disclosed wherein the nitrogen atom is part of a non-aromatic ring, or where the three alkyl groups are identical.
WO 03/035708 seeks to provide a cocatalyst component for use in combination with olefin polymerization catalyst precursors, particularly metallocenes that result in highly active polymerization catalysts with good storage stability. The cocatalysts disclosed are composed of an ammonium cation and a NCA and are supported on a fine particle carrier. The substituents used in the ammonium cation are selected from hydrogen, alkyl and arylalkyl. The NCA is of a similar type as disclosed e.g. in U.S. Pat. No. 5,416,177 discussed above. It is also said that the substituents may form a ring with each other, but no specific examples or suggestions of such compound are made.
There is still a need in the art for new and improved catalyst systems having good activity for the polymerization of olefins, in order to achieve specific polymer properties, such as high melting point, high molecular weights or to increase conversion without deteriorating the resulting polymer's properties. Also, in propylene polymerization increased propylene conversion often goes along with a decrease in molecular weight.
It is therefore an object of the present invention to provide a process and a catalyst system for use in a process for the polymerization of olefins, especially propylene, wherein the resulting polymers have a high molecular weight and/or a high melting point. Ideally, such catalyst system should also exhibit high catalytic activity under (propylene) polymerization conditions.
It is also an object of the present invention to provide a process and a catalyst system for use in a process for polymerizing olefins, especially propylene, which process and catalyst system allow raising olefin conversion rates, without substantially lowering the molecular weight of the resulting polymer, thus making commercial polymerization processes more efficient.